Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already cover the safety profile (readOnlyHint, destructiveHint=false, openWorldHint), and the description goes well beyond them by disclosing block semantics ('latest block'), return units, the 400/uncaught-error path, and cost model ($0.003/call via x402 USDC or card key, first 25 free, failed calls free). It also implicitly explains the idempotentHint=false annotation by noting each call is charged. Minor gaps remain around finality/reorg handling and response shape.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Purpose and scope are front-loaded in the first clause, followed by routing guidance, then error/pricing detail. Every sentence carries operational information; none is filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a single-parameter read tool with no output schema, the description covers everything an agent needs: what is read, on which chain, at which block, in which units, how to route away to siblings, and what it costs. Nothing material is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and the single parameter is self-explanatory, so baseline is 3. The description adds value beyond the schema by supplying a concrete example address and the validation rule (malformed address is rejected with 400 rather than erroring server-side), making the accepted input format unambiguous.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a precise verb+resource+scope: reading native ETH balance of a 0x address on Arbitrum One at the latest block, returned in wei and ETH. It also explicitly distinguishes itself from siblings arb_token_balance (ERC-20) and wallet_portfolio (all holdings), so an agent can pick it without opening other schemas.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives a concrete use case ('check whether a wallet can pay gas or a transfer') and names the two alternatives with the conditions that select them. It additionally documents failure behavior (malformed address rejected with 400, uncharged) and billing, covering when the call is worth making.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.